Enhancement of the immune response using CD36-binding domain

ABSTRACT

The invention relates to reagents and methods for enhancing an immune response using CD36 binding region/antigen hybrid polypeptides or polynucleotides encoding the hybrid polypeptides.

RELATED APPLICATIONS

This application claims priority to U.S. Ser. No. 60/341,771 filed Dec. 12, 2001, which is hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to reagents and methods for enhancing an immune response using CD36 binding region/antigen hybrid polypeptides or polynucleotides encoding the hybrid polypeptides.

BACKGROUND

Cells undergoing programmed cell death (i.e. apoptosis) are recognized by phagocytes such as macrophages and immature dendritic cells (Albert, M. L., et al., “Immature dendritic cells phagocytose apoptotic cells via alpha v beta 5 and CD36, and cross-present antigens to cytotoxic T lymphocytes,” J. Exp. Med. 188(7):1359 (1998)). This recognition leads to the uptake and degradation of the dying cells.

Some of the molecular details of this recognition are now known. For example, recognition of and adherence to apoptotic cells by phagocytes occurs by several mechanisms including a CD36-dependent mechanism (Albert, M. L., et al., (1998)). CD36 is a cell surface glycoprotein that is expressed on dendritic cells, monocytes, and macrophages (Platt, N., et al., Proc. Natl. Acad. Sci., 93: 12456 (1996)). Furthermore, CD36 is a receptor for thrombospondin. (Asch et al. “Isolation of the thrombospondin membrane receptor,” J. Clin. Invest., 79:1054 (1987)).

Thrombospondins are a family of extracellular matrix adhesive proteins. Thrombospondin 1 (TSP 1) inhibits angiogenesis and modulates endothelial cell motility, adhesion, and cell growth. Thrombospondin 1 has multiple functional domains, including a type I repeat which has homology with properdin (Arch, A. S., et al., Biochem. Biophys. Res. Commun., 182(3):1208 (1992); Crombie, R., et al., J. Exp. Med., 187(1): 25 (1998); Magnetto, S., et al., Cell Biochem. Funct, 16(3): 211 (1998); Carron, J. A., et al., Biochem. Biophys. Res. Commun. 270(3): 1124 (2000); and Li, W., et al., J. Biol. Chem., 268:16179 (1993)). Within the type I repeat are two CSVTCG (SEQ ID NO.: 11) sequences that serve as binding sites for CD36. (Pearce, S. F. A., et al., “Recombinant GST/CD36 fusion proteins define a thrombospondin binding domain: evidence for a single calcium-dependent binding site on CD36,” J. Biol. Chem. 270: 2981 (1995); and Asch, A. S., et al., “Thrombospondin sequence motif (CSVTCG; SEQ ID NO.: 11) is responsible for CD36 binding,” Biochem. Biophys. Res. Commun. 182: 1208 (1992)).

Engulfment of apoptotic bodies by phagocytes, including dendritic cells, is mediated by CD36 and results in cross-presentation of antigens to cytotoxic T-lymphocytes. (Albert M. L., et al. (1998)). Furthermore, human monocyte-derived macrophages phagocytose apoptotic neutrophils and eosinophils through a thrombospondin/CD36 dependent mechanism. (Stern et al., “Human monocyte-derived macrophage phagocytosis of eosinophils undergoing apoptosis. Mediation by alpha v beta 3/CD36/thrombospondin recognition mechanisms and lack phlogistic response,” Am. J. Pathol. 149(3):911 (1996)). Therefore, CD36 and thrombospondin play an important role in the recognition and phagocytosis of apoptotic cells.

Classical vaccine technology has included the development of both live and inactivated vaccines. Live vaccines are typically attenuated non-pathogenic versions of an infectious agent that are capable of priming an immune response directed against a pathogenic version of the infectious agent. In recent years there have been advances in the development of live recombinant vaccines (e.g., recombinant poxviruses) in which foreign antigens of interest are expressed from a viral vector. Although there are numerous examples of live vaccines that are very effective (e.g., vaccinia in the eradication of smallpox), there are inherent risks associated with live vectors. For example, it is possible to contaminate live vaccines with harmful adventitious agents, since live vaccines cannot be subjected to harsh inactivation or purification procedures. Additionally, there is a possibility when using live vaccine vectors, of “runaway vaccination” causing systemic viremia in immunocomprised recipients. Finally, live viral vectors elicit strong inflammatory and immunogenic responses against vector components that limit the utility of repeat administration.

Inactivated vaccines are comprised of killed whole pathogens, or soluble proteins or protein subunits. While generally considered safe, the efficacy of inactivated vaccines at eliciting broad, long-lasting responses is of concern for some vaccine preparations. In fact, most inactivated vaccines fail to produce a significant CD8+ cellular response necessary for cytolytic immune activity. Recombinant proteins are promising inactive vaccine or immunogenic composition candidates because they can be produced at high yield and purity. However, recombinant proteins can be poorly immunogenic. Therefore, there is a need for methods and compositions that enhance the immune response to inactivated vaccines, especially vaccines containing recombinant proteins.

Adjuvants have been used for many years to enhance the immune response to antigens present in vaccines, including subunit or component vaccines comprised of recombinant proteins. Currently, alum is the most commonly used adjuvant for human administration. However, although its efficacy has been established, it is ineffective for certain vaccinations (e.g. influenza vaccination) and inconsistently elicits an immune response with certain immunogens. Therefore, there remains a need for safe, effective, and easily manufactured compositions and methods for enhancing an immune response.

Recently, nucleic acid vectored vaccines (NAVAC) have been developed. These vaccines involve direct inoculation of an organism with nucleic acid vectors containing inserts encoding antigens (i.e. genetic vaccination). It appears that both the method (e.g. intramuscular injection, epidermal Gene Gun-mediated administration) and route of inoculation (e.g. intramuscular, epidermal, mucosal) are important in determining the efficacy of the immune response for NAVAC (Robinson et al., Vaccine 11:957 (1993); Ulmer et al., Vaccine 15:792 (1997); and Shiver et al., Vaccine, 15:884 (1997)). This has led to inconsistent stimulation of an immune response using NAVAC. Therefore, there remains a need for effective methods for using NAVAC to consistently elicit a strong immune response.

Despite our increasing understanding of the complex cellular and molecular interactions involved in certain aspects of an immune response, there remains a need for improved methods and compositions, such as vaccines, for enhancing an immune response. More specifically, there remains a need to develop methods and reagents that utilize our growing understanding of antigen presenting cell-recognition molecules, such as CD36, and pathways that depend on these molecules, such as CD36/thrombospondin-dependent pathways.

SUMMARY OF THE INVENTION

The present invention provides improved methods for immunizing a host against an antigen. In one embodiment, an isolated chimeric nucleic acid molecule encoding a chimeric polypeptide is provided. In a preferred embodiment, the chimeric nucleic acid molecule comprises a nucleic acid sequence encoding at least one CD36 binding domain and another nucleic acid sequence encoding at least one immunogenic amino acid sequence (i.e., antigen). Following introduction of the nucleic acid molecule into a host, a chimeric polypeptide is expressed resulting in an immune response against the immunogenic amino acid sequence. Thus, in one embodiment, the present invention provides an isolated nucleic acid molecule encoding an immunogenic chimeric polypeptide, the nucleic acid molecule being suitable for administration to a host, either alone or as a pharmaceutical composition. In another embodiment, the present invention provides an immunogenic chimeric polypeptide suitable for administration to a host, either alone or as a pharmaceutical composition. In yet another embodiment, a method of immunizing a host using the isolated nucleic acid molecule or the chimeric polypeptide is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a restriction map (FIG. 1A), the nucleotide sequence (FIGS. 1B-J; see also GenBank Accession Number X14787) and the amino acid sequence (FIG. 1K) of mature thrombospondin 1 (TSP1). The signal sequence is found at amino acids 1-31 and the connecting peptide at amino acids 32-44; the CD36 binding domains are found at amino acids 504-511 and 447-452; and, the beta sheet region is found at amino acids 453-463.

FIG. 2 illustrates the structure (FIG. 2A) and nucleotide as well as amino acid sequence (FIG. 2B) of the chimera pVITHROMBgp120FU. In FIG. 2B, The N-terminus of the thrombo=gp120 hybrid polypeptide begins at the methionine residue encoded by the codon adjacent to the Pst I site.

FIG. 3 shows V3 CTL activity following a single immunization with control plasmid (open squares), plasmid gp120 (filled diamonds), plasmid thrombo=gp120 (filled triangles) or vP1008 (open squares).

FIG. 4 shows V3 CTL activity three weeks after a second immunization given three weeks after a first immunization, with control plasmid (open squares), plasmid gp120 (filled diamonds), plasmid thrombo=gp120 (filled triangles) or vP1008 (open squares).

FIG. 5 shows anti-gp160 antibody production following immunization during weeks 0 and 3 with plasmid thrombo=gp120. Individual sera from each mouse was diluted 1:400 and assayed by kinetics ELISA (KELISA) using recombinant HIV MN/BRU envelope glycoprotein as antigen.

FIG. 6 shows V3 activity three weeks after a second immunization with control plasmid (open squares), plasmid gp120 (filled diamonds), plasmid thrombo=gp120 (filled triangles) or vP1008 (open squares).

FIG. 7 shows V3 (gp120) CTL activity at seven weeks after a second immunization with control plasmid (open squares), plasmid gp120 (filled diamonds), plasmid thrombo=gp120 (filled triangles) or vP1008 (open squares).

FIG. 8 shows anti-gp160 antibody responses to gp120 following two successive immunizations with control plasmid (vical), plasmid gp120 (vical-gp120), plasmid thrombo=gp120 or vP1008 after a first immunization (week 0) and a second immunization (week 3). KELISA results for three mice from each group are graphed individually (open squares, closed triangles, and closed diamonds). Antibody responses were calculated at the time of immunization, and at 3 weeks, 6 weeks, 8 weeks, and 10 weeks after the first immunization.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a chimeric nucleic acid molecule comprising a nucleic acid sequence encoding one or more CD36 binding regions positioned in-frame to one or more nucleic acid sequences encoding one or more immunogenic amino acid sequences (i.e., antigens). Such as nucleic acid molecule is referred to as a chimeric or hybrid nucleic acid. The nucleic acid sequence(s) encoding the one or more CD36 binding region(s) may be positioned upstream (5′) or downstream (3′) from the nucleic acid sequence(s) encoding the one or more antigens, provided a chimeric polypeptide comprising both the CD36 binding region and antigenic region is expressed in cells transfected with the chimeric nucleic acid molecule. Preferably, the nucleic acid sequence(s) encoding the one or more CD36 binding region(s) is ligated upstream from the nucleic acid(s) encoding the one or more antigens. In preferred embodiments, the chimeric nucleic acid molecule is admixed in a pharmaceutically acceptable carrier to form the immunogenic compositions of the current invention. In other preferred embodiments, the chimeric polypeptide is admixed in a pharmaceutically acceptable carrier to form the immunogenic compositions of the current invention. Recombinant DNA methods used herein are generally those set forth in commonly used references such as Sambrook et al., Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory Press, 1989) and/or Current Protocols in Molecular Biology (Ausubel et al., Eds., Green publishers inc. and Wiley and Sons 1994). All references cited in this application are expressly incorporated by reference herein.

The term “CD36 binding region” means a polypeptide sequence capable of binding to the cell surface glycoprotein CD36 in solid-phase binding assays (see, for example, Crombie et al., J. Exp. Med., 187: 25 (1998); and Pearce et al., J. Biol. Chem. 270: 2981 (1995)). Typically, the CD36 binding region includes sequences that do not bind CD36 in the absence of other sequences within the CD36 binding region. The boundaries of the CD36 binding region are typically defined by identifying the amino or carboxy-most polypeptide subsegments within the CD36 binding region that are capable of binding CD36 or enhancing binding to CD36 in solid-phase binding assays. In a preferred embodiment, the CD36 binding region of the present invention has the amino acid sequence of the CD36 binding region of TSP1 (SEQ ID NO.: 7; FIGS. 1B-K; GenBank Accession No. X14787). For the purposes of practicing the present invention, this region preferably comprises amino acids 447-463 ligated with or without intervening amino acids to amino acids 504-511 of TSP1.

Within the CD36 binding region are CD36 binding domains, which are subsegments of the CD36 binding region that independently bind CD36 in solid-phase binding assays. Binding of the CD36 binding region to the cell surface glycoprotein CD36 occurs through one or more of these CD36 binding domains. The CD36 binding domain typically comprises the amino acid sequence CSVTCG (SEQ ID NO.: 11) or sequences related thereto. A sequence related to the sequence CSVTCG (SEQ ID NO.: 11), or “consisting essentially of” CSVTCG (SEQ ID NO.: 11) is a sequence that has at least 4 of the amino amino acids in CSVTCG (SEQ ID NO.: 11), or comprises conservative substitutions of at least 4 of the amino acids in CSVTCG (SEQ ID NO.: 11), and retains the ability to bind CD36 in solid-phase binding assays. Representative CD36 binding domains are found at amino acids 447-452 and amino acids 504-511 of TSP1 (FIGS. 1B-K).

In one embodiment, the CD36 binding region contains at least two CD36 binding domains. The domains are typically separated by spacer regions (or “spacer amino acids”) that preferably have a beta sheet conformation. Methods are known in the art for predicting whether a polypeptide region will have a beta sheet conformation based on the primary sequence of the polypeptide. In addition, the conformation of polypeptide regions can be determined experimentally using well-known methods. The beta sheet spacers may be of any length provided that the resultant CD36 binding region enhances an immune response. It is preferred that the spacers are approximately 11 amino acids in length. It is more preferred that the spacer be 11 amino acids in length. In a most preferred embodiment, the spacer is identical to amino acids 453-463 of TSP1 or has the sequence DGVITRIRLCN (FIGS. 1B-K). In a preferred embodiment, the CD36 binding region contains two CD36 binding domains separated by a spacer region of about 11 amino acids.

The nucleic acid sequence(s) encoding the one or more antigens encodes an amino acid sequence that causes an immune response within a host upon expression within the host. It is preferred that, following expression of an effective amount of chimeric nucleic acid or administration of an effective amount the chimeric polypeptide, the host is immunized against the antigen. An “effective amount” is that which enhances an immune response as measured using assays known in the art including, for example, antibody assays, antigen specific cytotoxicity assays, and assays measuring the expression of cytokines. In preferred embodiments, the chimeric nucleic acid molecule or chimeric polypeptide functions as a vaccine. As is understood in the art, a vaccine is an immunogenic composition containing an antigen that, when administered to an animal, stimulates an immune response that at least partially protects the animal from challenge by a cell or organism expressing the antigen. “Partially protects the animal” means that the immunogenic composition elicits an antibody-based or cytotoxic T lymphocyte response against the antigen.

For example, where the antigen is a tumor antigen, the host is preferably protected from the development of a tumor and/or the host acquires the ability to eliminate an existing tumor from the body. The term “tumor antigen” as used herein includes both tumor associated antigens (TAAs) and tumor specific antigens (TSAs), where a cancerous cell is the source of the antigen. A TAA is an antigen that is expressed on the surface of a tumor cell in higher amounts than is observed on normal cells or an antigen that is expressed on normal cells during fetal development. A TSA is an antigen that is unique to tumor cells and is not expressed on normal cells. The term tumor antigen includes TAAs or TSAs, antigenic fragments thereof, and modified versions that retain their antigenicity.

Suitable TAA or TSA may include wild-type or mutated antigens such as, for example, gp100 (Cox et al., Science, 264:716-719 (1994)), MART-1/Melan A (Kawakami et al., J. Exp. Med., 180:347-352 (1994)), gp75 (TRP-1) (Wang et al., J. Exp. Med., 186:1131-1140 (1996)), tyrosinase (Wolfel et al., Eur. J. Immunol., 24:759-764 (1994)), NY-ESO-1 (WO 99/18206) melanoma proteoglycan (Hellstrom et al., J. Immunol., 130:1467-1472 (1983)), antigens of MAGE family (i.e., MAGE-1, 2, 3, 4, 6, and 12; Van der Bruggen et al., Science, 254:1643-1647 (1991)), antigens of BAGE family (Boel et al., Immunity, 2:167-175 (1995)), antigens of GAGE family (i.e., GAGE-1,2; Van den Eynde et al., J. Exp. Med., 182:689-698 (1995)), antigens of RAGE family (i.e., RAGE-1; Gaugler et at., Immunogenetics, 44:323-330 (1996)), N-acetylglucosaminyltransferase-V (Guilloux et at., J. Exp. Med., 183:1173-1183 (1996)), p15 (Robbins et al., J. Immunol. 154:5944-5950 (1995)), β-catenin (Robbins et al., J. Exp. Med., 183:1185-1192 (1996)), MUM-1 (Coulie et al., Proc. Natl. Acad. Sci. USA, 92:7976-7980 (1995)), cyclin dependent kinase-4 (CDK4) (Wolfel et al., Science, 269:1281-1284 (1995)), p21 ras (Fossum et at., Int. J. Cancer, 56:40-45 (1994)), BCR-abl (Bocchia et al., Blood, 85:2680-2684 (1995)), p53 (Theobald et al., Proc. Natl. Acad. Sci. USA, 92:11993-11997 (1995)), p185 HER2/neu (Fisk et al., J. Exp. Med., 181:2109-2117 (1995)), epidermal growth factor receptor (EGFR) (Harris et al., Breast Cancer Res. Treat, 29:1-2 (1994)), carcinoembryonic antigens (CEA) (Kwong et al., J. Natl. Cancer Inst., 85:982-990 (1995) U.S. Pat. Nos. 5,756,103; 5,274,087; 5,571,710; 6,071,716; 5,698,530; 6,045,802; EP 263933; EP 346710; and, EP 784483); carcinoma-associated mutated mucins such as MUC-1 gene products (Jerome et al., J. Immunol., 151:1654-1662 (1993)); EBNA gene products of EBV, for example, EBNA-1 gene product (Rickinson et al., Cancer Surveys, 13:53-80 (1992)); E7, E6 proteins of human papillomavirus (Ressing et al., J. Immunol, 154:5934-5943 (1995)); prostate specific antigens (PSA) (Xue et al., The Prostate, 30:73-78 (1997)); prostate specific membrane antigen (PSMA) (Israeli, et al., Cancer Res., 54:1807-1811 (1994)); idiotypic epitopes or antigens, for example, immunoglobulin idiotypes or T cell receptor idiotypes (Chen et al., J. Immunol., 153:4775-4787 (1994)); KSA (U.S. Pat. No. 5,348,887), NY-ESO-1 (WO 98/14464), and NY-BR-1 (WO 01/47959). Other suitable tumor antigens are known in the art.

Similarly, where the antigen is derived from an infectious agent such as a bacterium, virus, parasite or fungus, the host is preferably protected from infection by an organism expressing the antigen and/or the host acquires the ability to eliminate an existing infection. For example, where the antigen is the HIV gp120 protein or an immunogenic fragment derived therefrom, the host is preferably protected from infection by HIV and/or the host acquires the ability to eliminate HIV already existing in the host.

The chimeric nucleic acid molecule may further include a nucleic acid sequence encoding an immunostimulatory molecule. Many suitable immunostimulatory molecules are available to one of skill in the art including, for example, cytokines (e.g., interleukin-2 (IL-2), interleukin-12 (IL-12), and granulocyte-macrophage colony stimulating factor (GM-CSF)), co-stimulatory molecules (e.g., the B7 family of molecules such as B7.1 and B7.2), and/or other lymphokines that enhance the immune response. In certain embodiments of the current invention, the immunogenic polypeptide composition of the current invention includes both a CD36 binding/immunogen chimeric polypeptide and an immunostimulatory molecule, in admixture with a pharmaceutically acceptable diluent or carrier.

It is preferred that the nucleic acid sequences encoding the CD36 binding region and the antigen be operably linked so as to be expressed in a cell as a chimeric polypeptide. In one embodiment, the CD36 binding region(s) may be coupled to the amino terminus or carboxy terminus of the antigen coding region. Alternatively, the CD36 binding region may be coupled to a region within the antigen, provided that at least one immunologically effective epitope (i.e., an epitope capable of eliciting an immune response) and preferably all immunologically effective epitopes of the antigen are preserved. In a preferred embodiment, the fusion construct is constructed by ligating a polynucleotide encoding a CD36 binding region in frame to a polynucleotide encoding the antigen and expressing the ligated polynucleotide in a cell transfected with the polynucleotide.

It is also possible to generate the chimeric polypeptide by linking the CD36 binding region to the antigen indirectly through either a covalent or non-covalent association. Preferably, the linkage between the CD36 binding region and the antigen is covalent. Methods of performing such linkages are known in the art (see, for example, “Chemistry of Protein Conjugation and Crosslinking”. 1991, Shans Wong, CRC Press, Ann Arbor). Suitable crosslinking agents known in the art include, for example, the homobifunctional agents glutaraldehyde, dimethyladipimidate and bis(diazobenzidine) and the heterobifunctional agents m-maleimidobenzoyl-N-hydroxysuccinimide and sulfo-m maleimidobenzoyl-N-hydroxysuccinimide. Other suitable agents are known in the art.

In one embodiment, indirect linkage of the CD36 binding region may be accomplished using a binding pair. For these embodiments, the CD36 binding region is coupled to a first binding pair member that is capable of binding under physiological conditions to a second binding pair member that is coupled to the antigen. Many suitable binding pairs, such as biotin and avidin, are known in the art. Preferably, the CD36 binding region is coupled to the amino terminus of the antigen-coding region.

In certain embodiments, the CD36 binding region/antigen chimeric polypeptides of the current invention are secreted from cells. It is preferred that the polypeptides are expressed in an immature form containing a signal sequence, which is cleaved during the secretion process. The signal sequence may be derived from either a prokaryote or a eukaryote. For example, a prokaryotic signal sequence may be used where eukaryotic post-translational modifications of the fusion polypeptide are not necessary to elicit an enhanced immune response against the antigen. For these situations, the chimeric polypeptides may be expressed and secreted in vitro in a bacterial host cell.

In certain embodiments, the signal sequence comprises an endoplasmic reticulum (ER)-targeting sequence and a signal peptidase (SP) sequence. Inclusion of these sequences typically results in secretion of the CD36 binding region/antigen chimeric polypeptides from cells transfected with a CD36 binding region/antigen chimeric polynucleotide of the current invention. In a preferred embodiment, the ER/SP sequences comprise amino acid residues 1-44 of TSP1 as shown in SEQ ID NO.: 7 and FIGS. 1B-K. Other ER/SP sequences and methods for identifying such sequences are known in the art (Goa et al., “Aids Res. and Human Retroviruses, 10: 1359 (1994)). In preferred embodiments, the ER/SP sequence is positioned 5′ of the one or more nucleic acid sequences encoding a CD36 binding region and the one or more nucleic acid sequences encoding the antigen.

In another aspect, the present invention provides an isolated form of the chimeric CD36 binding/immunogen polynucleotide of the present invention, wherein a nucleic acid sequence encoding a CD36 binding region is ligated in frame to a nucleic acid sequence encoding an antigen. In one embodiment, the present invention provides a chimeric polynucleotide including a secretion signal sequence ligated upstream of the CD36 binding sequence which is ligated upstream of the antigen-coding sequence.

By “isolated” nucleic acid is intended a nucleic acid molecule, DNA or RNA, which has been removed from its native environment. Isolated DNA molecules include recombinant DNA molecules maintained in heterologous host cells or purified (partially or substantially) DNA molecules in solution. Isolated RNA molecules include in vivo, or in vitro RNA transcripts of the DNA molecules of the present invention. Isolated nucleic acid molecules according to the present invention further include such molecules produced synthetically. Recombinant DNA molecules contained in a vector are also considered isolated for the purposes of the present invention. The isolated nucleic acid molecules and expression vectors of the present invention may be constructed using standard recombinant techniques widely available to one skilled in the art. Such techniques may be found in common molecular biology references such as Molecular Cloning: A Laboratory Manual (Sambrook, et al., 1989, Cold Spring Harbor Laboratory Press), Gene Expression Technology (Methods in Enzymology, Vol. 185, edited by D. Goeddel, 1991. Academic Press, San Diego, Calif.), and PCR Protocols: A Guide to Methods and Applications (Innis, et al. 1990. Academic Press, San Diego, Calif.).

In preferred embodiments, the isolated nucleic acids of the current invention include a transcriptional regulatory region that is operatively-linked to the chimeric nucleic acid molecule. By “operatively linked” is meant transcription of the chimeric polynucleotide is affected by the activity of the transcriptional regulatory region. Preferably, the transcriptional regulatory region drives high-level gene expression in the target cell. The transcriptional regulatory region may comprise, for example, a promoter, enhancer, silencer, repressor element, or combinations thereof. A wide variety of promoters can be utilized for the current invention. Suitable transcriptional regulatory regions include, for example, the CMV promoter (i.e., the CMV-E1 promoter shown in FIGS. 2A and 2B); the SV40 late promoter; promoters from eukaryotic genes, such as the estrogen-inducible chicken ovalbumin gene, the interferon genes, the gluco-corticoid-inducible tyrosine aminotransferase gene, and the thymidine kinase gene; and the major early and late adenovirus gene promoters. Furthermore, as a large number of retroviruses are known that infect a wide range of eukaryotic host cells, the long terminal repeats (LTRs) frequently may also suffice as transcriptional regulatory regions. It is also possible to operably link the chimeric polynucleotide to a tissue- or cell-specific transcriptional regulatory region to affect expression of the chimeric polynucleotide in certain cells or tissues. As such, when the isolated CD36 binding/antigen polynucleotide is inserted into a cell, transcription of the chimeric polynucleotide is induced resulting in expression of the CD36 binding/antigen chimeric polypeptide.

The chimeric polynucleotides are preferably administered as part of an expression vector. The vectors can be either RNA or DNA, either prokaryotic or eukaryotic, and are typically of viral or plasmid origin. As such, the chimeric polynucleotide is inserted into an expression vector such that the polynucleotide is expressed when transformed into a host cell.

Expression vectors of the present invention include any vectors that function (i.e., direct gene expression) in recombinant cells of the present invention, including in bacterial, fungal, parasite, insect, animal, and plant cells. Preferred expression vectors of the present invention can direct gene expression in animal cells, preferably mammalian, most preferably human cells.

There are many methods and vectors available for expressing polynucleotides in cells. For instance, several types of mammalian expression systems are known in the art. (See e.g., Sambrook et al., “Expression of Cloned Genes in Mammalian Cells.” In Molecular Cloning: A Laboratory Manual, 2nd ed. (1989)). Viral transduction methods may comprise the use of a recombinant DNA or an RNA virus comprising a nucleic acid sequence encoding the chimeric polypeptide to infect a cell, resulting in expression of the chimeric polypeptide. Suitable viral vectors include, for example, adenovirus, pox viruses (i.e., vaccinia or avipox), polio virus, and alphavirus, among others known in the art.

Examples of suitable retroviral vectors include, but are not limited to Moloney murine leukemia virus (MoMuLV), Harvey murine sarcoma virus (HaMuSV), murine mammary tumor virus (MuMTV), SIV, BIV, HIV and Rous Sarcoma Virus (RSV). A number of additional retroviral vectors can incorporate multiple genes. Since recombinant retroviruses are defective, they require assistance in order to produce infectious vector particles. This assistance can be provided, for example, by using helper cell lines that contain plasmids encoding all of the structural genes of the retrovirus under the control of regulatory sequences within the LTR. These plasmids are missing a nucleotide sequence which enables the packaging mechanism to recognize an RNA transcript for encapsitation. Helper cell lines which have deletions of the packaging signal include but are not limited to Ψ2, PA317 and PA12, for example.

In a preferred embodiment, the viral vector is a poxvirus such as vaccinia virus (Smith, et al., 1983, Gene, 25 (1): 21-8; Moss, 1992, Biotechnology, 20: 345-62; Moss, 1992, Curr. Top. Microbiol. Immunol., 158: 25-38; U.S. Pat. Nos. 5,364,773, 5,990,091, and 5,174,993). In certain embodiments, a highly attenuated strain of vaccinia, designated MVA, may be used as a vector (U.S. Pat. No. 5,185,146). A preferred vector is the NYVAC vector (U.S. Pat. Nos. 5,364,773 and 5,494,807). In other preferred embodiments, the poxvirus vector is ALVAC (1) or ALVAC (2), both of which are derived from canarypox virus (see, for example, U.S. Pat. Nos. 5,833,975 and 5,990,091; Tartaglia, et al., J. Virol. 67: 2370 (1993)). Fowlpox virus is another avipoxvirus that may also be used in practicing the present invention (see, for example, U.S. Pat. No. 5,766,599). Other suitable poxvirus vectors are known in the art.

In certain embodiments, the vector is a plasmid vector. Many plasmid expression vectors are known in the art and could be used with the current invention. In preferred embodiments, isolated nucleic acids are directly administered to an animal, virtually any expression vector that is effective in animal cells can be used. Preferred vectors where the isolated nucleic acids of the current invention are intended for NAVAC applications.

Bacterial vectors may also be used with the current invention. These vectors include, for example, Shigella, Salmonella, Vibrio cholerae, Lactobacillus, Bacille Calmette Guérin (BCG), and Streptococcus (See e.g., WO 88/6626; WO 90/0594; WO 91/13157; WO 92/1796; and WO 92/21376). In these bacterial vector embodiments of this invention, a chimeric CD36 binding/immunogen polynucleotide of the invention may be inserted into the bacterial genome, may remain in a free state, or may be carried on a plasmid (as described above). Other suitable vectors include the E. coli expression vector pUR278 and the glutathione S-transferase (GST) vector pGEX.

Other delivery techniques including DNA-ligand complexes, adenovirus-ligand-DNA complexes, direct injection of DNA, CaPO₄ precipitation, gene gun techniques, electroporation, liposomes and lipofection (Mulligan, R., 1993, Science, 260 (5110): 926-32) have also been demonstrated to be useful. Lipofection may be accomplished by encapsulating an isolated DNA molecule within a liposomal particle and contacting the liposomal particle with the cell membrane of the target cell. Liposomes are artificial membrane vesicles which are useful as delivery vehicles in vitro and in vivo. It has been shown that large unilamellar vesicles (LUV), which range in size from 0.2-4.0 μm can encapsulate a substantial percentage of an aqueous buffer containing large macromolecules. RNA, DNA and intact virions can be encapsulated within the aqueous interior and be delivered to cells in a biologically active form. Other suitable methods are available to one skilled in the art, and it is to be understood that the present invention may be accomplished using any of the available methods of transfection.

The current invention further provides isolated CD36 binding/immunogen chimeric polypeptides. The term “isolated” as used herein refers to the removal of a polypeptide from its natural environment and does not imply any specific level of purity of the polypeptide. Many methods are known in the art that can be used to prepare the CD36 binding/immunogen chimeric polypeptides. For example, the fusion polypeptides may be prepared as recombinant fusion polypeptides using the CD36 binding/immunogen expression polynucleotides and recombinant cell lines. The CD36 binding/immunogen chimeric polypeptides may also be prepared by covalently linking the CD36 binding region to the antigen using chemical cross-linking methods and cross-linking agents well-known in the art as described above. (see, for example, “Chemistry of Protein Conjugation and Crosslinking”. 1991, Shans Wong, CRC Press, Ann Arbor).

In certain embodiments, the isolated polypeptides of the current invention include additional purification fusion polypeptide segments that assist in purification of the polypeptides. Suitable fusion segments include, among others, metal binding domains (e.g., a poly-histidine segment), immunoglobulin binding domains (e.g., Protein A; Protein G; T cell; B cell; Fc receptor or complement protein antibody-binding domains), sugar binding domains (e.g., a maltose binding domain), and/or a “tag” domain (e.g., at least a portion of α-galactosidase, a strep tag peptide, a T7 tag peptide, a Flag peptide, or other domains that can be purified using compounds that bind to the domain, such as monoclonal antibodies). Other suitable fusion segments are well known in the art.

In another aspect, the current invention includes recombinant cells and cell lines that express the CD36 binding/immunogen fusion polypeptides of the current invention. The recombinant cells and cell lines may be prokaryotic or preferably eukaryotic cells, that are transformed with one or more CD36 binding/immunogen expression vector. A cell can be “transformed,” as the term is used in this specification, with a nucleic acid molecule, such as a recombinant expression vector, by any method by which a nucleic acid molecule can be introduced into the cell. Transformation techniques include, but are not limited to, transfection, infection, electroporation, microinjection, lipofection, adsorption, and protoplast fusion. Transformation may be stable or transient. A “cell line” refers to any immortalized recombinant cell of the present invention that is not a transgenic animal. Transformed nucleic acid molecules of the present invention can remain extrachromosomal or can integrate into one or more sites within a chromosome of the transformed (i.e., recombinant) cell in such a manner that their ability to be expressed is retained.

Suitable host cells include any cell that can be transformed with a nucleic acid molecule of the present invention, but are preferably a host cell from an organism to which an expressed CD36 binding/antigen chimeric polypeptide will be administered. Many such cells are available to the skilled artisan including, for example, primary cells such as fibroblasts or dendritic cells, Vero cells, non-tumorigenic mouse myoblast G8 cells (e.g., ATCC CRL 1246), K562 erythroleukemia cells, mouse NIH/3T3 cells, other fibroblast cell lines (e.g., human, murine, or chicken embryo fibroblast cell lines), myeloma cell lines, Chinese hamster ovary cells, LMTK31 cells, and/or HeLa cells. In one embodiment, the recombinant cell line is a myeloma cell line employing immunoglobulin promoters operatively linked to the chimeric polynucleotides of the current invention.

A recombinant cell of the current invention is preferably produced by transforming a host cell with one or more recombinant molecules, each comprising one or more chimeric polynucleotides of the current invention operatively linked to an expression vector containing one or more transcription control sequences, examples of which are disclosed herein. A recombinant cell of the present invention includes any cell transformed with at least one of any chimeric polynucleotide of the present invention. In a preferred embodiment of this aspect of the invention, the transformed host cells are immortalized mammalian cell lines capable of expressing high levels of CD36 binding/antigen chimeric polypeptides.

Recombinant DNA technologies can be used to improve expression of transformed nucleic acid molecules by manipulating, for example, the number of copies of the nucleic acid molecules within a host cell, the efficiency with which those nucleic acid molecules are transcribed, the efficiency with which the resultant transcripts are translated, and the efficiency of post-translational modifications. Recombinant techniques useful for increasing the expression of chimeric polynucleotides of the present invention include, but are not limited to, operatively linking the polynucleotides to high-copy number plasmids, integration of the polynucleotides into one or more host cell chromosomes, addition of vector stability sequences to plasmids, substitutions or modifications of transcription control signals (e.g., promoters, operators, enhancers), substitutions or modifications of translational control signals (e.g., ribosome binding sites, Shine-Dalgarno sequences), modification of nucleic acid molecules of the present invention to correspond to the codon usage of the host cell, deletion of sequences that destabilize transcripts, and use of control signals that temporally separate recombinant cell growth from recombinant protein production during fermentation.

In another aspect, the current invention provides a method for producing a CD36 binding/antigen fusion polypeptide in a host cell. For this method, an isolated chimeric CD36 binding/antigen polynucleotide is introduced into an expression vector to produce a CD36 binding/antigen expression vector. The CD36 binding/antigen expression vector is then introduced into a host cell to produce a transformed host cell. The transformed host cell is then maintained under conditions suitable for the expression of CD36 binding/antigen chimeric polypeptide. Finally, the fusion polypeptide is collected.

Preferred host cells are described above. Effective culture conditions include, but are not limited to, effective media, bioreactor, temperature, pH, and oxygen conditions that permit protein production. An effective medium refers to any medium in which a cell is cultured to produce a fusion polypeptide of the present invention. Such medium typically comprises an aqueous medium having assimilable carbon, nitrogen and phosphate sources, and appropriate salts, minerals, metals and other nutrients, such as vitamins. Cells of the present invention can be cultured in conventional fermentation bioreactors, shake flasks, test tubes, microtiter dishes, and petri plates. Culturing can be carried out at a temperature, pH, and oxygen conditions appropriate for a recombinant cell. Such culturing conditions are within the expertise of one of ordinary skill in the art. Depending on the vector and host system used for production, resultant polypeptides of the present invention can remain within the recombinant cell, be secreted into the fermentation medium or into a space between two cellular membranes (e.g., the periplasmic space in E. coli), or be retained on the outer surface of a cell or viral membrane.

The phrase “collecting the peptide”, as well as similar phrases, refers to collecting the whole medium containing the fusion polypeptide and need not imply additional steps of separation or purification. Fusion polypeptides of the present invention can be purified using a variety of standard protein purification techniques, such as, but not limited to, affinity chromatography, ion exchange chromatography, filtration, electrophoresis, hydrophobic interaction chromatography, gel filtration chromatography, reverse phase chromatography, concanavalin A chromatography, chromatofocusing, and differential solubilization. Fusion polypeptides of the present invention are preferably retrieved in “substantially pure” form. As used herein, “substantially pure” refers to a purity that allows for the effective use of the protein as a therapeutic composition or diagnostic. A therapeutic composition for animals, for example, should exhibit no substantial toxicity and preferably should be capable of stimulating the production of antibodies in a treated animal.

The chimeric nucleic acids and polypeptides of the present invention may be administered to a host alone or in combination with another agent. In certain embodiments, the chimeric nucleic acid molecule or polypeptide is administered in a pharmaceutically acceptable carrier or diluent. Many pharmaceutically-acceptable carriers and diluents as well as methods for determining the route and quantities for administration are known in the art.

The term “pharmaceutically acceptable” means approved by a regulatory agency of the Federal or a state government or listed in the U.S. Pharmacopeia or other generally recognized pharmacopeia for use in animals, and more particularly in humans. The terms “carrier” and “carrier or diluent” refers to a diluent, adjuvant, excipient, or vehicle with which the therapeutic is administered. Carriers are well known in the art. For example, saline solutions and aqueous dextrose and glycerol solutions can be employed as liquid carriers, particularly for injectable solutions. Examples of suitable pharmaceutical carriers are described in “Remington's Pharmaceutical Sciences” by E. W. Martin.

In embodiments of the current invention wherein a polynucleotide is administered to an animal, certain preferred formulations are those used in NAVAC applications. The polynucleotide may be used in a naked/free form, free of any delivery vehicles. When administered in a naked/free form the polynucleotide can be simply diluted in a physiologically acceptable solution (such as sterile saline or sterile buffered saline) with or without a carrier. When present, the carrier preferably is isotonic, hypotonic, or weakly hypertonic, and has a relatively low ionic strength (i.e., a sucrose solution containing, for example, 20% sucrose).

Alternatively, the polynucleotide can be associated with agents that assist in cellular uptake (i.e., delivery vehicles). Delivery vehicles include, but are not limited to, anionic liposomes, polynucleotide and/or non-polynucleotide vector components, cationic lipids, microparticles, (e.g., gold microparticles), precipitating agents (e.g., calcium phosphate)) or any other transfection-facilitating agent.

In certain preferred embodiments, an immunogenic composition of the current invention is co-administered with an adjuvant. Typically, adjuvants used with the current invention are non-toxic and do not cause undesirable side effects. Examples of adjuvants that can be used with the current invention include, but are not limited to, aluminum hydroxide and aluminum phosphate, collectively commonly referred to as alum.

It is preferred that the chimeric nucleic acid or polypeptide induces or enhances the immune response of the host against the antigen. To accomplish this, an effective amount of an immunogenic composition comprising a chimera is administered to the host. Standard techniques may be used to determine an effective amount of the chimera to be administered. In particular, an effective amount may be determined by techniques well-known to those skilled in the medical or veterinary arts taking into consideration such factors as the immunogenicity of the antigen, the condition of the animal intended for administration (i.e., the weight, age, and general health of the animal), the mode of administration, and the type of formulation. The amount of immunogenic composition as well as a dosage regime may be adjusted to provide the optimum induction of an immune response. In preferred embodiments, the host is a mammal, most preferably a human.

Suitable routes of administration are many, as is known in the art. Such routes include, for example, mucosal (e.g., ocular, intranasal, oral, gastric, pulmonary, intestinal, rectal, vaginal, or urinary tract), parenteral (e.g., subcutaneous, intradermal, intramuscular, intravenous, or intraperitoneal), or intranodal routes. The administration may be by injection, oral administration, inhalation, transdermal application, rectal administration, or any other route of immunization that enables the modulation of an animal's immune system. In certain preferred embodiments, the administration is by injection. Certain preferred routes are those that are effective for NAVAC applications, such as intramuscular, and most preferably skin. The administration can be achieved in a single dose or repeated at intervals.

A particularly preferred method of administering the immunogenic compositions of the current invention is by a prime-boost protocol. Typically, an initial administration of a chimeric polynucleotide or polypeptide composition followed by a boost with the same immunogenic composition, will elicit an enhanced immune response (See e.g., WO 98/58956). Timing of the booster following the prime may be determined by a skilled artisan to provide optimum response.

The following examples describe and illustrate the methods and compositions of the invention. These examples are intended to be merely illustrative of the present invention, and not limiting thereof in either scope or spirit. Unless indicated otherwise, all percentages and ratios are by weight. Those skilled in the art will readily understand that variations of the materials, conditions, and processes described in these examples can be used.

Example 1 Preparation of Polynucleotide Constructs

This Example describes the preparation of a DNA construct encoding a chimeric polypeptide wherein the mature gp120 form of HIV-1 env is ligated to two CD36 binding domains of thrombospondin, and the insertion of the construct into an expression vector. The sequence for TSP1 was obtained from the National Center for Biotechnology Information (NCBI) (Genbank Accession # X14787; FIGS. 1B-K). As previously described, the TSP1-CD36 binding domains have been clearly defined as type 1 properdin-like repeats defined by the amino acid sequence CSVTCG. The antigen used in this Example is HIV gp120(mn) (Goa, F. et al., Aids Res. and Human Retroviruses, 10: 1359 (1994).

The chimeric polypeptide was constructed such that the TSP1 endoplasmic reticulum targeting signal and CD36 binding domain(s) were positioned at the NH₂-terminal end, such that the secreted polypeptide would retain the CD36 binding domains. Because the TSP1-CD36 binding domains are small (i.e., 6 amino acids) and the interaction with CD36 is believed to be conformation dependent, the binding domains were engineered to be expressed as in native TSP1 to retain proximal structural conformation.

The coding sequences for the engineered portion of the fusion protein were generated using overlapping oligonucleotides synthesized by Operon Technologies (Alameda, Calif.). The amino acid sequences of human TSP1 that were incorporated into the chimeric construct are shown below (numbers refer to amino acids in human TSP1):

-   -   TSP1 1-31: coding sequences for signal sequence     -   TSP1 32-44: coding sequences for signal peptidase cleavage     -   TSP1 447-452: coding sequences generated CD36 Binding Domain 1     -   TSP1 453-463: coding sequences for a Beta Sheet region     -   TSP1 504-511: CD36 Binding Domain 2

The joining Beta Sheet region between the CD36 binding domains of thrombospondin was retained in the construct because protein sequence analysis revealed both TSP1-CD36 domains were contained in regions of strong Beta Sheet conformation. To maintain conformational integrity, the spacer region engineered between Domain 1 and 2 also contained Beta Sheet conformation.

The TSP1 portion of the hybrid molecule was synthesized by overlapping and amplifying the oligonucleotides shown below:

HTHROM1A (SEQ ID NO.: 2) ATCATCCTGCAGATGGGGCTGGCCTGGGGACTAGGCGTCCTGTTCCTGATG CATGTGTGTGGCACCAACCGCATTCCAGAG HTHROM2 (SEQ ID NO.: 3) AGACCCCTTGCGGGCGGCCCCGGTGAGTTCAAAGATGTCAAACACGCTGTT GTCTCCGCCAGACTCTGGAATGCGGTTGGTGC HTHROM3 (SEQ ID NO.: 4) GGGGCCGCCCGCAAGGGGTCTTCTTGTTCTGTGACATGTGGTGATGGTGTG ATCACAAGGATCCGGCTCTGCAAC HTHROM4 (SEQ ID NO.: 5) ATCATCGGTACCCCATAATAGACTGTGACCCACAATTTTTCGCTCCCTCCT CCACAGGTGACAGAACAGTTGCAGAGCCGGATCCTTG

Primers HTHROM1A and HTHROM2 overlap one another and primer HTHROM3 overlaps HTHROM2 and HTHROM4. Primer HTHROM3 overlaps primer HTHROM2 and HTHROM4. To generate the full coding sequence of the TSP1 portion of the hybrid, the primers were subjected to three minutes at 95° C. (denaturation), three minutes at 50° C. (annealing) and 10 minutes at 72° C. (extension). The resultant fragment representing the full-length product was gel purified and further purified by PCR using primers HTHROM1A and HTRHOM4 for 26 cycles at 94° C. for 30 seconds followed by 72° C. for 60 seconds. The resulting fragment was cloned and confirmed to be correct by DNA sequencing.

Primer HTHROM1A contains a Pst I restriction site (CTGCAG) and primer HTHROM4 contains an Asp718 restriction site (GGTACC). These were used to position the TSP1 portion of the chimera upstream and in-frame with the HIV gp120 portion of the hybrid (the antigen) as shown below.

The HIV gp120 coding sequences used for the engineered fusion were assessed using published methods to identify the ER targeting signal sequence so it could be eliminated from the sequences used to express gp120 in the final fusion construct (Goa et al., 1994). Signal sequences in gp120 were identified as amino acids 1-21. These amino acids were not included in the chimeric construct. For fusions where CD36 binding domains are upstream of the antigen of interest, it is essential to remove all intervening cleavage sequences (i.e., signal peptidase sequences) between the CD36 binding domains and the antigen of interest.

For cloning purposes, the TSP1 portion of the hybrid was manipulated using the Pst I and Asp718 restriction sites. This 250 bp Pst-I Asp718 fragment containing the engineered CD36 binding domains, including sequences encoding 13 amino acids of the 5′end of gp120 (up to Asp718 restriction site), was directionally cloned using the Pst I/Asp718 sites into a prepared eukaryotic expression vector for NAVAC.

The engineered fragment was inserted into the expression vector in the proper orientation and in-frame with the Asp718 restriction site in the gp120 sequences. This resulted in a continuous reading frame from the TSP1 signal through the CD36 binding domains terminating at the gp120 3′ carboxy terminus. The sequence was confirmed by DNA sequencing to ensure retention of the appropriate reading frame. The construct was designated thrombo=gp120 (FIGS. 2A and 2B). Large quantities of purified thrombo=gp120 were prepared using Qiagen Giga Preparations (endotoxin free) as per manufacturer instructions for NAVAC (Qiagen Inc. Valencia, Calif.).

Example 2 Analysis of Immunogenicity

This Example provides an analysis of the immunogenicity of thrombo=gp120 in mice. Mice were immunized once by an intramuscular (i.m.) route with 100 ug of DNA plasmid thrombo=gp120 expressing the TSP1-CD36 binding domain-gp120 fusion, plasmid vical gp120 expressing only gp120, or control plasmid in 0.1 ml of PBS. As a positive control mice were immunized with a recombinant poxvirus vP1008 (NYVAC-env; U.S. Pat. No. 5,494,807). Mice received two immunizations three weeks apart.

Both cell-mediated immunity (i.e. cytotoxic T-Cell (CTL) activity) and humoral antibody responses were analyzed in the study. For CTL activity, pooled spleen cells of three mice from each experimental group were restimulated in vitro with naive syngeneic spleen cells infected with vp1008 (HIV MN env). After 6 days, the spleen cells were assayed against P815 target cells pulsed with V3 epitope peptide (peptide 121). Specific cytotoxicity was calculated as the difference in cytotoxicity between peptide-pulsed P815 targets and P815 targets with no peptide pulse. Data for the CTL analysis is expressed as percent specific lysis (i.e. difference between HIV epitope peptide pulsed and un-pulsed targets) at various effector:Target (E:T) ratios. This analysis of CTL utilized Chromium-51 release assays (reviewed in Brunner, et al., “Quantitative assay of lytic action of immune cells on Cr-51 labeled allogenic target cells in vivo; inhibition by isoantibody and by drugs,” Immunology, 14:181 (1968)).

Humoral antibody responses were analyzed by bleeding three mice from each group at the appropriate time point. Serum samples were collected at appropriate timepoints from representative mice of both test and control groups, using a retroorbital plexis procedure (McGuill, M. W. and Rowan A. N. Biological Effects of Blood Loss: implications for sampling volumes and techniques. I.L.A.R. News 31: 4 (1989)) The initial bleed (week 0) was prior to the initial immunization and the (week 3) bleed was just prior the second immunization. Individual sera from each mouse was diluted 1:400 and assayed by kinetics ELISA for antibodies against recombinant HIV MN/BRU envelope glycoprotein (PMS-France) (Cox, et al., J. Natl. Cancer Inst. 71:973 (1983); and Holbrook, et al. Cancer Res. 43:4019 (1983)).

FIGS. 3 and 4 summarize results of CTL analysis for this experiment. Three weeks after the initial immunization, CTL activity was not detected in mice immunized with control plasmid containing no gp120 sequences or vical gp120. In contrast, strong CTL responses were generated by mice immunized with DNA plasmid thrombo=gp120 (FIG. 3). The CTL responses elicited by this plasmid exceeded CTL responses generated by an optimal dose of vP1008 (NYVAC-env) given by the optimal intraperitoneal route. Three weeks after the second immunization, mice immunized with plasmid gp120 still had no detectable CTL activity (FIG. 4). Mice immunized with thrombo=gp120 maintained the strong CTL response seen after the first immunization.

Analysis of antibody response induced by the experimental samples confirmed that thrombo=gp120 constructs were effective at eliciting an immune response against gp120. Anti-HIV envelope glycoprotein antibody assays were performed on samples collected at 0, 3, 4, and 7 weeks after the start of the experiment (i.e. time of initial immunization). The strongest, most reliable responses were generated by mice immunized with thrombo-gp120 and recombinant poxvirus vp1008 (FIG. 5). In fact, no antibody responses were detected in mice after one immunization with control plasmid gp120. However, all three mice immunized one time only with thrombo=gp120 generated an antibody response. Antibody levels were similar to levels generated after immunization one time with vP1008.

Example 3 Further Analysis of Immunogenicity

This Example provides an analysis of the immunogenicity of thrombo=gp120 in mice that is a repeat of the analysis of Example 2 carried out for longer time periods. Immunizations and CTL and antibody assays were performed as described in Example 2. For this experiment, CTL analysis was performed at both 3 weeks and 7 weeks after the second immunization.

Three weeks after the second immunization, mice immunized with plasmid vical gp120 had no CTL activity above control levels (FIG. 6, Table I). However, mice immunized with thrombo=gp120 generated excellent CTL activity comparable to that generated with vP1008. Seven weeks after the second immunization mice immunized with vical gp120 showed a week CTL response (FIG. 7, Table II). However, mice immunized with thrombo=gp120 showed a strong CTL response.

TABLE I Specific Lysis (CTL) three weeks after primary immunization TEST ARTICLE E:T % SPECIFIC LYSIS Negative control plasmid 40:1 −5.4 Negative control plasmid 20:1 −0.6 Negative control plasmid 10:1 0.5 Negative control plasmid  5:1 −0.7 Gp120 40:1 −1.0 Gp120 20:1 2.3 Gp120 10:1 1.2 Gp120  5:1 1.4 Thromb = gp120 40:1 8.1 Thromb = gp120 20:1 13.1 Thromb = gp120 10:1 10.0 Thromb = gp120  5:1 0.7 VP1008 positive control 40:1 17.7 VP1008 positive control 20:1 10.5 VP1008 positive control 10:1 4.1 VP1008 positive control  5:1 1.5

TABLE II Specific CTL lysis, three and seven weeks after the second immunization TEST ARTICLE E:T % LYSIS WK 3 % LYSIS WK 7 Negative control plasmid 40:1 8.9 0 Negative control plasmid 20:1 1.0 3.0 Negative control plasmid 10:1 −0.1 2.6 Negative control plasmid  5:1 −0.6 0.1 gp120 40:1 9.0 12.4 gp120 20:1 4.1 4.6 gp120 10:1 5.9 5.3 gp120  5:1 1.6 4.1 Thromb = gp120 40:1 59.1 58.4 Thromb = gp120 20:1 53.3 47.6 Thromb = gp120 10:1 44.6 38.8 Thromb = gp120  5:1 31.9 28.5 vP1008 positive control 40:1 40.3 64.4 vP1008 positive control 20:1 26.3 64.4 vP1008 positive control 10:1 15.7 40.2 vP1008 positive control  5:1 7.6 27.5

Antibody measurements confirmed that thrombo=gp120 induced a strong immunological response against gp120. All three mice immunized with thrombo=gp120 generated a strong antibody response after only one immunization (FIG. 8). In fact, when averages of three mice for each experimental group were calculated, highest titers of anti-gp120 antibodies were obtained following the initial immunization with either DNA plasmid thrombo=gp120 or the recombinant poxvirus vector pV1008 (Table III).

Antibody responses following the second immunization were also evaluated (week 4, 6, 8, 10 of FIG. 8 and Table III). The strong antibody response of mice immunized with plasmid thrombo=gp120 continued to escalate after the second immunization and continued throughout the study to show a stronger antibody titer than positive control vP1008.

TABLE III Kinetics ELISA (mOD/min) WEEK Test article 0 3 4 6 8 10 Negative control 1 1 1.6 0.6 1 1 gp120 1 3 22.7 29.3 27 24 Thromb = gp120 1.7 16 36.3 46.3 50 52.6 vP1008 1 8 21.6 29 31 29

The results of Examples 2 and 3 demonstrate that significant enhancement of both a cell-mediated and humoral immune response against HIV gp120, is obtained by coupling HIV gp120 with the TSP1-CD36 binding domain. Not to be limited by theory, it is believed that these results are related to the proposed mode of immunological enhancement, (i.e., APC Targeting), as other similar constructions expressing the identical version of gp120 as a fusion with other various targeting domains have failed to enhance responses. These results suggest that the resultant immunological enhancement is not simply associated with non-specific effects of altered expression and or persistence (i.e., half-life) of gp120 expressed as a fusion product.

This method of enhancing immune responses by targeting the antigen of interest to APC's can be applied to virtually any immunological target of interest, including those important for both infectious, and neoplastic diseases.

Throughout this application, various patents, publications, books, and nucleic acid and amino acid sequences have been cited. The entireties of each of these patents, publications, books, and sequences are hereby incorporated by reference into this application. 

What is claimed is:
 1. An isolated nucleic acid molecule encoding an immunogenic chimeric polypeptide, the nucleic acid molecule comprising at least a first nucleic acid sequence positioned upstream of and in frame with a second nucleic acid sequence, wherein: the first nucleic acid sequence encodes two domains comprising the amino acid sequence CSVTCG (SEQ ID NO: 11) separated by a spacer region of about 11 amino acids; and, the second nucleic acid sequence encodes at least one tumor antigen or an antigen derived from an infectious agent.
 2. The isolated nucleic acid molecule of claim 1 wherein the spacer region has a beta sheet conformation.
 3. The isolated nucleic acid molecule of claim 2 wherein the spacer amino acids comprise DGVITRIRLCN (SEQ ID NO: 1).
 4. The isolated nucleic acid molecule of claim 1 further encoding a signal nucleic acid sequence consisting essentially of an endoplasmic reticulum signal sequence and a signal peptidase cleavage sequence.
 5. The isolated nucleic acid molecule of claim 1 wherein the infectious agent is selected from the group consisting of a bacterium, a parasite, a virus, a fungus, and a cancerous cell.
 6. The isolated nucleic acid molecule of claim 5 wherein second nucleic acid sequence encodes a gp120 polypeptide or immunogenic fragment thereof.
 7. The isolated nucleic acid molecule of claim 1 or 5 further comprising a transcriptional regulatory region operably linked to at least one of the nucleic acid sequences.
 8. The isolated nucleic acid molecule of claim 7 wherein said transcriptional control region is the CMV promoter.
 9. An immunogenic composition for generating an immune response in a host, the composition comprising a nucleic acid molecule of claim 1 or
 5. 10. An immunogenic composition for generating an immune response in a host, the composition comprising a nucleic acid molecule of claim
 6. 11. An immunogenic composition for generating an immune response in a host, the composition comprising a nucleic acid molecule of claim
 7. 12. An immunogenic composition for generating an immune response in a host, the composition comprising a nucleic acid molecule of claim
 8. 13. The nucleic acid molecule of claim 1 wherein the first nucleic acid sequence encodes at least the amino acid sequence of SEQ ID NO.:
 10. 